Influence of Mn Doping Content on Magnetic Properties of (Mn, N)

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Influence of Mn doping content on magneticproperties of (Mn, N) co-doped ZnO system

H.B. Ruan, C.Y. Kong, G.P. Qin, W.J. Li, T.Y. Yang,F. Wu, L. Fang

PII: S0304-8853(14)00546-0DOI: http://dx.doi.org/10.1016/j.jmmm.2014.06.024Reference: MAGMA59151

To appear in: Journal of Magnetism and Magnetic Materials

Received date: 5 September 2012Revised date: 24 December 2013

Cite this article as: H.B. Ruan, C.Y. Kong, G.P. Qin, W.J. Li, T.Y. Yang, F. Wu, L.Fang, Influence of Mn doping content on magnetic properties of (Mn, N) co-doped ZnO system, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/ 10.1016/j.jmmm.2014.06.024

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Influence of Mn doping content on magnetic properties of

(Mn, N) co-doped ZnO systemH.B. Ruan1,2,3, C.Y. Kong1, , G.P. Qin1,3, W.J. Li3, T.Y. Yang1, F. Wu2, L. Fang3,

1Key Laboratory of Optoelectronic Functional Materials, Chongqing Normal University,

Chongqing 400030, Peoples Republic of China

2 Chongqing Key Laboratory of Micro/Nano Material Engineering and Technology, Chongqing

University of Arts and Sciences, Chongqing 402160, Peoples Republic of China

3Department of Applied Physics, Chongqing University, Chongqing 400044, Peoples Republic

of China

Keywords: (Mn, N) co-doped ZnO, ferromagnetism, magnetic moment, first-principles

calculations

Abstract

In this paper, by investigating the magnetic properties of (Mn, N) co-doped ZnO films with

various Mn doping contents, we found that the total magnetic moments of the samples would

reach a limit under a certain N doping condition even if more Mn ions were incorporated into the

ZnO lattice. Based on first-principles calculations, we propose that the unique magnetic

interactions between Mn2+ dopants in the presence of nitrogen and the very low solubility of

substitutional N relative to Mn in ZnO are responsible for the experimental observation.

Corresponding author: Electronic mail: [email protected] (C.Y. Kong). Electronic mail: [email protected] (L. Fang).


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1. Introduction

Diluted magnetic semiconductors (DMSs), which involve the charge and spin degrees of

freedom of electrons in a single substance, are considered as the most promising candidate for

spintronic devices [1]. In the quest for materials with a highT C , Zn1- xMn xO systems are attracting

much more attention, since the theoretical study predicted that room temperature (RT)

ferromagnetism might exist in Mn doped p-type conducting ZnO in terms of the Zener model [2].

According to first principles calculations, in the absence of p-type doping, Mn doped ZnO would

exhibit antiferromagnetic (AFM) properties while ferromagnetism can be stabilized only by hole

doping [2-8]. As ZnO is intrinsicallyn-type conducting and doping Mn2+ into ZnO does not itself

introduce additional carriers, p-type defects must be introduced deliberately by other routes, e.g.,

through N substitution for O [9]. Recently, in spite of the experimental results concerning the

existence of ferromagnetism in Zn1- xMn xO is still under debate, many groups have reported high

T C ferromagnetism in Zn1- xMn xO films by codoping N [9-13]. Additionally, a ferromagnetic (FM)

ground state in (Mn, N) co-doped ZnO has also been demonstrated by several theoretical

calculations [4-8]. However, so far, the influence of Mn doping content on the magnetic behavior

of Zn1- xMn xO:N system has been little noticed. In particular, the solubility of Mn is much larger

than that of nitrogen in ZnO [15-16], thus more investigations are necessary to clarify the

magnetic interactions between the doped Mn ions in Zn1- xMn xO:N with different Mn contents. In

this paper, both the experiments and first-principles calculations suggest that the magnetic

moments of Zn1- xMn xO:N system will reach a limit under a certain N doping condition even if

more Mn ions are incorporated into the ZnO lattice.


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2. Experiments

The Zn1- xMn xO targets for sputtering were prepared using a solid state reaction method.

ZnO (99.99%) and MnO (99.99%) powders mixed according to the nominal atomic ratios were

mixed thoroughly and sintered at 1000 C for 10 h in air. The Zn1- xMn xO thin films were

deposited on fused silica substrates by RF magnetron sputtering. The Mn concentration of the

Zn1- xMn xO films was determined to be 1.82 %, 3.56 %, and 6.72 % with a relative error of 10%

by using the energy dispersive x-ray spectroscopy. The thickness of the films was about 500 nm.

In order to obtain a flat nitrogen profile, a four-folded ion implantation was performed at 300 K

with N+ ions of energy 10 keV (dose of 21015cm-2), 30 keV (dose of 51015cm-2), plus 70 keV

(dose of 91015cm-2), and 150 keV (dose of 2.61016cm-2). This resulted in a ~500 nm deep

box-shaped region with N concentration ~91020 cm-3. Then all the samples were annealed at

500 C for 60 min under a flowing N2 ambient to activate the implanted ions and recover the

crystallinity. X-ray diffraction (XRD) pattern reveals that all the films before and after

implantation are single phase and have wurtzite structure with c-axis preferred orientation.

Raman scattering measurements were performed in the backscattering geometry using the 514.5

nm line of an Ar + laser as excitation source. The carrier concentration of the films was

investigated by van der Pauw Hall measurements at RT. The magnetic properties were measured

by using a superconducting quantum interference device magnetometer (Quantum Design,

MPMS-XL). The magnetic field was applied parallel to the film plane.


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3. Results and discussions

It is well known that the Raman scattering study is a versatile and powerful technique to

understand the structural changes, lattice defects, and any secondary phase in DMSs. Figure

1shows the Raman spectra of the annealed Zn1- xMn xO:N films with different Mn doping contents.

We observed two normal modes at 437 and 575 cm-1 corresponding to E2 (high) and A1 (LO),

respectively [17]. The existence of E2 (high) mode indicates that all the films keep a good

wurtzite ZnO structure. This result is well consistent with that of XRD. Besides the normal

modes of vibration of ZnO, four additional modes at 275 (P1), 510 (P2), 525 (P3) and 645 (P4)

cm-1 were also observed. According to previous studies, peaks at 275, 510 and 642 cm-1 are

attributed to N-related defect while the peak at 525 cm-1 is assigned to the being local vibration

of Mn in ZnO [18-19]. Besides these modes, no trace of manganese metal, oxides, or any binary

nitrides related vibration modes is observed in any of the samples. All these results indicate that

both Mn and N ions are incorporated into the ZnO lattice successfully.

The carrier concentration of Mn mono-doped and (Mn, N) co-doped ZnO films with various

Mn doping contents is shown in figure 2. The results indicate that Mn doping significantly

suppresses the electron concentration of the Zn1-xMnxO films, suggesting an effective

compensation of the intrinsicn-type carriers. Our result is consistent with the report by Hlaing

Oo et al . [ 20], which is understandable that the incorporation of substitutional Mn could suppress

the formation of native shallow donor defects such as oxygen vacancies and/or Zn interstitials in

ZnO [20-21]. After N+-implantation, due to the significant compensation defects and/or

localization of holes, the as-implanted Zn1- xMn xO:N films still keepn-type conductivity.


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Post-annealing is needed to remove these defects and to activate N atoms by moving them to the

right lattice sites. When the films were followed by annealing at 500 C for 60min in an N2

ambient, it is seen that the carriers in the 3.56 % and 6.72 % Mn-doped films are inverted from

negative to positive, evidence of a hole-dominant transport mechanism, whereas the film with a

1.82% Mn content remainsn-type conductive. This difference might be due to the fact that there

exist different degrees of compensation by background donors in Zn1- xMn xO:N films with

various Mn doping contents. The carrier type of the samples has also been confirmed by the

thermoelectric probe measurements.

Figure 3 shows the magnetic-field dependence of magnetization (M-H ) at 300K for the Mn

mono-doped, as-implanted, and annealed (Mn, N) co-doped ZnO films with a Mn concentration

of 3.56 %. The diamagnetic contribution from silica substrate has been subtracted from the raw

data. It can be seen that the Zn1- xMn xO ( x=3.56 %) film shows a nearly linear paramagnetic

behavior while clear hysteresis loops were revealed for the Zn1- xMn xO:N ( x=3.56 %) films,

suggesting that the incorporation of N plays a crucial role in establishing the ferromagnetic order

in Zn1- xMn xO system. This result is well consistent with other reports [8-13]. The bottom inset

shows theM-T curve for the annealed sample measured from 5 to 300 K, no transition from

ferromagnetic to paramagnetic state occurs, indicating that the Curie temperature is well above

room temperature. In addition, we also found that the saturation magnetic moment of the

annealed p-type Zn1- xMn xO:N ( x=3.56 %) films is 38.8 emu/cm-3, much larger than that (20.2

emu/cm-3) of the as-implantedn-type sample whose carrier concentration is about 21017 cm-3.

This seems that the stronger ferromagnetic order tend to exist in p-type sample with a


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hole-dominant transport mechanism. However, by comparing the RT ferromagnetic behavior of

the Zn1- xMn xO:N films with various Mn doping contents, as shown in figure 4, although there

exists distinct diversities including Mn doping contents, carrier type, and carrier concentration

for the three samples, interestingly, they exhibit comparative magnetic behavior. The results

indicate: i) the observed magnetic behavior has not direct correlations with the conductivity type

or the carrier concentration of the films, ii) under a certain N doping condition, the magnetic

moments of the Zn1- xMn xO:N system will reach a limit even if more Mn ions are incorporated

into the ZnO lattice.

According to Hall measurements, the average carrier concentration in our samples is around

1016-1017 cm-3, which is far below the threshold of an effective Ruderman-Kieeel-Kasuya-Yosida

(RKKY) model. Thus, although the carrier mediated RKKY-type FM coupling mechanism

presented by Dietlet al . [2] has been quite successful, it could not account for the origin of

observed ferromagnetism in the present samples. In our previous calculations [22], we found that

codoping N is a promising approach to enhance the ferromagnetic coupling between the

nearest-neighboring Mn ions due to the strong hybridization between the N 2 p and Mn 3d states,

in agreement with other reports [3-8]. Furthermore, we also found that the spin-polarized states

induced by N are so extended that they can mediate long-ranged FM exchange interactions

beyond the nearest-neighboring case. Hence we propose that both direct and indirect

double-exchange interactions are responsible for the dominant ferromagnetism in (Mn, N)

co-doped ZnO. Here the indirect double-exchange interaction is a natural extension of the well

known double-exchange effect but now mediated by the No orbitals. Our first-principles


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calculation showed that each No could introduce one localized orbital and simultaneously its

states overlap significantly with those of Mn2+ 3d near the Fermi level. The electron hoping from

the Mn2+

3d orbital to N 2 p empty orbital without changing spin direction would cause the Mn2+

ions connected via the No orbitals to align ferromagnetically. Here, it should be noted that, like

almost all of the literatures devoting to understand the magnetic interaction between the doped

magnetic ions in DMSs, our previous calculations were conducted only in the supercell

(Zn34Mn2O35 N) with two Mn atoms. However, under a certain N doping condition, if we further

increasing Mn atoms in the supercell, their magnetic coupling interactions are unknown.

In order to provide some insights into the question mentioned above, we employed the

Vienna ab inito simulation package (VASP) [23] to calculate the magnetic interactions between

the Mn ions in (Mn, N) co-doped ZnO system with more than two atoms in the supercell. The

plane wave basis set with the projector augmented plane wave method [24] was used with a

nonlocal correction in the form of the generalized gradient approximation [25]. As reported, Mn

(MnZn) and N (NO) dopants have a tendency toward staying close to each other with most stable

-O-Mn-N-Mn-O- complexes [4-8]. Furthermore, it was also suggested that the Mn ions tend to

cluster together as dimmers via an intervening O atom, rather than distribute themselves evenly

throughout the lattice [5-8]. Based on these results, we further added two Mn atoms beside the

-O-Mn-N-Mn-O- complex of a ZnO supercell containing 72 atoms. In table 1 we present the

energy differences between FM and AFM configuration for the various Mn substitutions in

Zn(Mnk )O:N (k= 4) systems. The calculations showed that the added Mn atoms preferred to exist

in AFM state energetically while the other two Mn atoms connected by an intervening N atom


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coupled together ferromagnetically. Thus in this case, the addition of Mn ions has no contribution

to improve the magnetic moments for Zn1- xMn xO:N system. On the other hand, Rao and Jena [26]

previously studied the structure and magnetism of N-capped Mn clusters. Based on their study

carried out on Mn clusters of sizes up to five Mn atoms, they proposed that N capping can be

used as a viable means to enhance both the stability and the FM coupling among Mn atoms in a

Mn cluster. The similar situation may also occur in Zn1- xMn xO:N system. Therefore, we further

calculated the local magnetic interaction between the Mn ions for thenMn-N (n=2, 3, 4) clusters

in ZnO the lattice. Again, our calculations indicate that if the amount of doped Mn atoms on the

neighbor N sites exceeds two of a tetrahedron in ZnO, the added Mn atoms have no contribution

to improve the magnetic moments for Zn1- xMn xO:N system. Interestingly, the simulation results

are consistent with the experimental results mentioned above. In contrast to the high solubility

(up to 35%) of Mn in ZnO, the solubility limit of N in ZnO is as low as ~1% [15-16]. Under a

certain N doping condition, if there have sufficient numbers of Mn atoms around each

substitutional N occur, the measured magnetic moments of the Zn1- xMn xO:N samples would

remain constant even if more Mn atoms are incorporated into the ZnO lattice. Therefore, due to

the unique local magnetic interactions between Mn2+ dopants in the presence of N and

simultaneously the very low solubility of substitutional N relative to Mn in ZnO, we conclude

that the magnetic moments will reach a limit of the (Mn, N) co-doped ZnO system. As is well

known, to obtain macroscopic FM ordering long ranged interactions are indispensable. Recently,

based on experiments several groups have attributed that long FM order in (Mn, N)-codoped

ZnO could be due to the formation of bond magnetic polaron (BMP) [9, 11-12], either direct


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overlap between BMPs or indirect BMP-impurity-BMP interactions. In the presence of Zn(Mn)O

systems by codoping of substitutional anionic ions such as C, N, and P, Lisenkov and co-workers

made a systematic theoretical study and suggested that the magnetic interactions in this case take

in form of a double exchange or a super-exchange between polarized tetrahedra each centered at

one of the condopants [7]. To verify this, we further added a substitute Mn-N-Mn complex in the

Zn34Mn2O35 N supercell and calculated its magnetic properties. Our total energy calculations

reveal that the system is ferromagnetic coupling energetically and the corresponding spin density

is displayed in figure 5. Therefore, our results support the magnetic mechanism proposed by

Lisenkov and co-workers. Under this condition, by using a simple percolation model proposed

by Herng [27], we derive that the lower bound of No content leads to macroscopic ferromagnetic

ordering in Zn1- xMn xO:N system is about 0.5 at.%. Until now, ferromagnetic mechanisms of

DMS are still not to be well understood, thus more work is needed to illustrate the fundamental

physical processes.

4. Conclusions

We have investigated the influence of Mn doping content on the magnetic properties of (Mn,

N) co-doped ZnO system by experiments and first-principles calculations. It is found that

although the substitute N+ ions play a crucial role in activating the ferromagnetism in Mn doped

ZnO, however, due to the very low solubility of the N relative to Mn and the unique magnetic

interactions between Mn2+ dopants in the presence of nitrogen in ZnO, the magnetic moments of

the Zn1- xMn xO:N system will reach a limit even if more Mn impurities are incorporated into the


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ZnO lattice.

Acknowledgments

This work was supported by the Natural Science Foundation of Chongqing Grant

(cstc2013jcyjA50031 and cstc2013jjB0023) and the National Natural Science Foundation of

China (Grant Nos. 11075314 and 50942021), and the Sharing Fund of Large-scale Equipment of

Chongqing University (Grant nos. 2012121559 and 2012121560).

References

[1] H. Ohno, Science 281(1998) 951.

[2] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287 (2000) 1019.

[3] K. Sato and H. Katayama-Yoshida, Physica E 10 (2001) 251.

[4] Q. Wang, Q. Sun, P. Jena, and Y. Kawazoe, Physical Review B 70 (2004) 052408.

[5] L. Zhao, P.F. Lu, Z.Y. Yu, X.T. Guo, Y. Shen, H. Ye, G.F. Yuan, and L. Zhang, Journal of

Applied Physics 108 (2010) 113924.

[6] M.H. N. Assadi, Y.B. Zhang, and S. Li, Journal of Physics: Condensed Matter 21 (2009)

185503.

[7] S. Lisenkov, A.N. Andriotis, R. Michael Sheetz, M. Menon, Physical Review B 83 (2011)

235203.

[8] W.S. Yan, Z.H. Sun, Q.H. Liu, Z.R. Li, T.F. Shi, F. Wang, Z.M. Qi, G.B. Zhang, and S.Q.

Wei, Applied Physics Letters 89 (2007) 242509.

[9] K.R. Kittistved, N.S. Norberg, and D.R. Gamelin, Physical Review Letters 94 (2005)

147209.


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[20]W.M. Hlaing Oo, L.V. Saraf, M.H. Engelhard, V. Shutthanandan, L. Bergman, J. Huso, and

M.D. McCiuskey, Journal of Applied Physics 105 (2009) 013715.

[21]M. Ivill, S.J. Pearton, Y.W. Heo, J. Kelly, A.F. Hebard, and D.P. Norton, Journal of Applied

Physics 101 (2007) 123909.

[22]H.B. Ruan, L. Fang, T.Y. Yang, G.P. Qin, W.J. Li, X.D. Meng, Y.H. Zhao, P. Bian, F. Wu, and

C.Y. Kong, Physica Status Solidi (b) (to be published).

[23]G. Kresse and J. Furthmuller, Physical Review B 54 (1996) 11169.

[24]P.E. Blochl, Physical Review B 50 (1994)17953.

[25]J.P. Perdew, and Y. Wang, Physical Review B 45 (1992)13244.

[26]B.K. Rao and P. Jena, Physical Review Letters 89 (2002) 185504.

[27]T.S. Herng, D.C. Qi, T. Berlijn, J.B. Yi, K.S. Yang, Y. Dai, Y.P. Yuan, I. Santoso, C.

Snchez-Hanke, X.Y. Gao, Andrew T.S. Wee, W. Ku, J. Ding, and A. Rusydi, Physical

Review Letters 105 (2010) 207201.


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Figures Captions:

Fig. 1. Raman spectra of Zn1- xMn xO:N films with vaious Mn doping contents.

Fig. 2. Carrier concentration of the Zn1- xMn xO and Zn1- xMn xO:N films with various Mn doping

contents. The p-type conductive samples are marked with dashed frame.

Fig 3. Room-temperatureM-H curves of ZnO:Mn, as-implanted and annealed Zn1- xMn xO:N

films with a Mn concentration of 3.56%. The inset shows theM-T curve of the annealed sample

under a magnetic field of 1kO.

Fig. 4. Room-temperatureM-H curves of Zn1- xMn xO:N films with various Mn contents.

Fig.5 . 3D isosurfaces of the average spin charge density of 0.02 e/3 for (a) Zn34Mn2O35N and

(b) Zn32Mn4O34N2 where red small balls represents O, green big balls stand s for Zn, and

dopants and labeled individually. Blue shells represent the 3D isosurfaces.


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Highlights

(Mn, N) codoped ZnO films with various Mn contents were prepared.

Room-temperature ferromagnetism was observed.

Magnetic moments reach a limit under a certain N doping condition.

Magnetic interactions between Mn dopans in the presence of N in ZnO were studied by

first-principles calculations.

Ferromagnetic coupling mechanism of (Mn, N) codoped ZnO has been discussed.

Table Captions:

Table 1 Energy differences E between various spin configurations of Zn(Mnk )O:N, k= 2, 3, 4.

Negative values of E indicate stability of the FM spin configuration. The asterisk indicates the

most stable spin configuration of eachk .


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F i g u r e

1

1 5 0

3 0 0

4 5 0

6 0 0

7 5 0

9 0 0

I n t e n s i t y ( a r b . u n i t )

R a m a n s h

i f t ( c m

- 1 )

P 1

P 2 P 4

P 3

x = 0

%

x = 3 . 5

6 %

x = 6 . 7

2 %

x = 1 . 8

2 %

E h i g h

2

A L O

1

Z n 1 -

x M n x

O : N

C o v e r

L e

t t e r


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F i g u r e

2

0

2

4

6

8

1 E 1 7

1 E 1 8

1 E 1 9

p - t y p e

M n m o n o -

d o p e

d Z n O

f i l m s

( M n ,

N ) c o - d o p e d

Z n O

f i l m s

M n c o n c e n

t r a t

i o n

( a t . % )

C a r r i e r c o n c e n t r a t i o n ( c m - )

p - t y p e


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F i g u r e

3

- 6 0 0 0

- 4 0 0 0

- 2 0 0 0

0

2 0 0 0

4 0 0 0

6 0 0 0

- 4 0

- 2 0 0 2 0 4 0

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0

0

2 8

3 0

3 2

3 4

3 6

A n n e a

l e d Z n 1

- x M n x

O : N

f i l m

x = 3 . 5

6 %

H = 1

k O e

M ( e m u / c m 3 )

T e m p e r a

t u r e

( K )

Z n 1

- x M n x

O

A s -

i m p l a n

t e d Z n 1

- x M n x

O : N

A n n e a

l e d Z n 1

- x M n x

O : N

M a g n e t i z a t i o n ( e m u / c m 3 )

H ( O e )

x = 3 . 5

6 %

T = 3

0 0 K


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F i g u r e

4

- 6 0 0 0

- 4 0 0 0

- 2 0 0 0

0

2 0 0 0

4 0 0 0

6 0 0 0

- 4 0

- 2 0 0 2 0 4 0

x = 1 . 8 2 %

x =

3 . 5 6 %

x =

6 . 7 2 %

M a g n e t i z a t i o n ( e m u / c m 3 )

H ( O e )

T = 3

0 0 K

Z n 1

- x M n x

O : N


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F i g u r e

5


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T

a b l e

1

S c h e m a t

i c b o n d

i n g

E F M E A F M

E ( m e V

)

2 M n

M n - N - M n

E ( ) * E ( )

- 2 0 0

3 M n

3 M n -

N

E ( ) E

( ) *

7 9

4 M n

M n -

O - M n - N - M n -

O - M n

E ( )

E ( )

2 4 6

M n -

N - M n - O - M n -

O - M n

E ( )

E ( )

2 2 8

E ( )

E ( )

2 4 3

M n -

N - M n - O - Z n -

O - M n -

O - M n

E ( )

E ( )

2 4 4

4 M n -

N

E ( ) E

( ) *

3 1 2

a b l e

Influence of Mn Doping Content on Magnetic Properties of (Mn, N)

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